1. Field of the Invention
The invention relates to hard particles suitable for incorporation in sintered alloys. The invention particularly relates to hard particles suitable for increasing the wear resistance of sintered alloys, to a wear-resistant iron-based sintered alloy that contains the hard particles, and to a method of producing this sintered alloy.
2. Description of Related Art
Sintered alloys having a ferrous matrix are conventionally used in, for example, valve seats. Hard particles can be incorporated into sintered alloys in order to further raise the wear resistance of the sintered alloy. The hard particles are generally incorporated into sintered alloys as follows. A powder of the hard particles is mixed into a powder having a low-alloy steel or stainless steel composition to obtain a mixed powder. A green compact is formed with this mixed powder. The green compact is subsequently sintered to make the sintered alloy.
Japanese Patent Application Publication No. 2001-181807 (JP 2001-181807 A) describes hard particles that contain, expressed as mass %, Mo: 20 to 60%, C: 0.2 to 3%, Ni: 5 to 40%, Mn: 1 to 15%, and Cr: 0.1 to 10% with the balance being Fe and unavoidable impurities. It is also stated here that, for example, Co may be added to these hard particles.
Using these hard particles, the adherence between the hard particles and the ferrous matrix that is the base material can be improved when a ferrous-matrix sintered alloy is produced. In addition, adhesive wear can be inhibited because an oxidation film is formed from the Mo at the hard particles.
The amount of Mo in solid solution in the hard particles can be raised by the addition of Ni to the hard particles described in JP 2001-181807 A. This functions to improve the oxidation characteristics of the added Mo and can thereby improve the wear resistance. In addition, Co has a low stacking fault energy and due to this the addition of Co to the hard particles can raise the hardness of the hard particles and improve the wear resistance. However, the moldability into the green compact can be impaired when the hardness of the hard particles has been raised prior to compaction by the addition of Co. Furthermore, this Ni and Co are more expensive than other elements, resulting in high raw material costs for hard particles to which Ni and/or Co has been added.
Considering these points, for example, ferromolybdenum (Fe—Mo—Si) hard particles have a low cost because they do not contain cobalt or nickel. In the case of ferromolybdenum (Fe—Mo—Si) hard particles, the hard particles themselves have a high hardness due to the incorporation of Si. However, an Si oxidation film is formed when ferromolybdenum is compacted with an iron-based powder as the matrix and sintered. The formation of the Si oxidation film may cause that solid-solution diffusion between the hard particles and the ferrous matrix during sintering is hindered as a consequence. The adhesive strength of the hard particles for the ferrous matrix may then be lowered and the wear resistance of the sintered alloy may be lowered. In addition, because oxidation of the Mo is inhibited by the oxidation of the Si, the formation of an oxidation film of Mo at the hard particle is impeded. As a result, adhesive wear may ultimately be promoted due to exposure of the iron due to rupture of the Si oxidation film during sliding.